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Find the value of x for which the function y = x\(^3\) - x has...

Mathematics
JAMB 1999

Find the value of x for which the function y = x\(^3\) - x has a minimum value.

  • A. \(-\sqrt{3}\)
  • B. \(-\frac{\sqrt{3}}{3}\)
  • C. \(\frac{\sqrt{3}}{3}\)
  • D. \(\sqrt{3}\)
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Correct Answer: Option C
Explanation

We are given the function:
\(y = x^3 - x\)

To find the critical points, we set the derivative equal to zero:
\(3x^2 - 1 = 0\)
Solving this gives:
\(x = \pm \frac{1}{\sqrt{3}}\)

Using the second derivative test, we find that:
\(\frac{d^2y}{dx^2} = 6x\)
- For \( x = \frac{1}{\sqrt{3}} \), \( \frac{d^2y}{dx^2} > 0 \) (minimum).
- For \( x = -\frac{1}{\sqrt{3}} \), \( \frac{d^2y}{dx^2} < 0 \) (maximum).

Thus, the function \( y = x^3 - x \) has a minimum value at:
\(x = \frac{1}{\sqrt{3}} \quad \text{or approximately} \quad x \approx 0.577\)


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